Dynamic testing of the hypothalamic pituitary system in familial hypogonadotropic hypogonadism has suggested the possibility of a central nervous disorder associated with variable pituitary secretory secretion. In order to investigate this issue further 7 patients of two families were studied. Male patients exhibited two types of FSH responses to LHRH administration. Two patients had a small response, and the other exhibited a normal response. The LH response was absent or small in both male and female patients. One month therapy with daily injections of LHRH did not improve pituitary responsiveness to a second LHRH test. Estrogen treatment given to females during one month had no improvement effect on another LHRH test. Estradiol benzoate produced a surge in both LH and FSH in half of patients tested which is surprising in the presence of a previous clomiphene negative response. Two of four women had elevated prolactin levels which increased defectuously after metoclopramide administration which could suggest an abnormal hypothalamic status. On the basis of these results it seems that familial hypogonadotropism is originated by an abnormality at two levels, the anterior pituitary and the hypothalamus as well.